Sloane brought 20 erasers and pens in total. Each pen costs $2 while each eraser costs $0.40. If he spent $32.50 in all, how many erasers did he buy?
There is no reasonable solution (unless you can buy fractions of pens and erasers).
Suppose Sloane bought everything using dimes (10 cent pieces).
No matter how many erasers he bought the cost of erasers would be an even number of dimes.
Similarly, no matter how many pens he bought the cost of pens would be an even number of dimes.
...but $32.50 represents an odd number of dimes.